The solar energy received each
year by Earth is roughly equal to 10,000 times the total energy
consumed by humanity. In other words, if we could only exploit
0,01% of this incoming solar energy for the profit of humankind,
we could do without oil, coal,
gas and uranium : there definitely lies the reason why it's been
a long time since men considered to get something significant
out of sunrays. Still, it's a long road between dreams and reality
: in 2002, solar energy represented 0,04%
of the global energy consumption in the world. Is it to say
that we will never be able to do better ? In order to enable everyone
to get to one's own conclusions, here are a couple of figures
related to this energy.

Could
we produce all french electricity with only solar panels ?

In 2002, the french electricity
production amounted to 550 TW.h (1 TW.h = 1 billion kW.h).

The annual production of a photovoltaic
panel is 100 kWh per m2, at least for the largest part of Europe
(below).

- that a majority of developped countries
are in zones equally or better insulated than France, that I
use as a reference for the calculation below (northern Europe
du Nord : Netherlands, United Kingdom, Germany, etc, are not
much below 100 kWh/m2/year ; only scandinavian countries are
much below that level),

- that all "developping countries"
are better insulated than France

If we only consider gross outputs,
we should cover 5,000,000,000 m2 to get the same amout of electricity,
that is 5.000 km2. It is indeed a lot in absolute figures, but
if we compare it to the total surface of France (which is over
500.000 km2), we see that it represents only 1% of the metropolitan
surface. And, most of all, the surface that is already built (excluding
roads) amounts to 10.000 km2 (see figures
on urbanization).

By covering only half of the existing
roof surface with solar panels, we could thus produce something
that is of the same magnitude than our present electricity consumption,
that is something which is between 20 and 40% of our total energy
consumption, depending on the way we calculate it (see explanations
here).

Furthermore, the total electricity
consumption in France in 2002 was not 550 TWh, but rather 420
TWh : 10% of the production is lost in the grid during transport
and distribution, and 15% is exported. As photovoltaic production
is generally made close to the place where electricity will be
used, we do not have to take the transportation losses into account
if this way to produce electricity becomes widespread. It would
also seem curious to produce electricity this way to export it,
hence it is rather for the 420 TWh of domestic consumption that
we should make our calculations, and that requires to cover "only"
0,8% of the country with panels.

But if such a way to produce electricity
became widespread, we would have to store a significant fraction
of the production. Indeed, there is sunlight only during the day
(it's even the definition of what a day is !), when an important
part of the electricity is consumed at night : consumption of
factories that operate all night, of fridges and freezers, of
part of domestic electronics, of lighting, heating and/or air
con....). Furthermore, there is more light in the summer, when
the electricity consumption, in most european countries, peaks
in the winter.

And that's where problems start,
because electricity is not easy to store.
Let's see, for example, what it would mean if we stored it using
hydrogen, this hydrogen being produced by electrolysis with the
electricity obtained from the solar panel :

Step

approximative
yield

Electrolysis to produce hydrogen

80%

Storing hydrogen in a tank under a 700 atmosphere
pressure

80% (that means
that 20% of the initial hydrogen is consumed - or its energy
equivalent - to compress the remaining 80%)

Liquefaction of hydrogen

50% (that means
that 50% of the initial hydrogen is consumed - or its energy
equivalent - to liquefy the remaining 50%)

Hence we see that it is rather
2,5% that 0,8% of the country that we should cover with solar
panels to take into acount the losses due to storage.

Then, if we want to know the real
net output, we must also deduce the initial energy investment
which is necessary to manufacture the solar panel, and the associated
storage device. Producing a solar panel consumes today the equivalent
of 5 years of production of the said panel. Then we must also
take into account the energy spent to manufacture the battery,
which is the only available storage device today, and the result
commonly given is that it takes roughly 10 years of production
to reimburse the initial energy investment of the total "panel
+ battery".

As a panel has a 25 year life
usage, we see that about half the gross production of the whole
device goes to the initial energy debt linked to the production
of the device. In other terms, if we want to take into account
the energy spending required to manufacture the device, it's rather
5% of France that we should cover with panels.

If we cannot, with such a percentage,
consider as possible to produce all our electricity with solar
panels, the above calculation still does not forbid to consider
that it should be possible to get much more out of these panels
than what we presently get.

But solar panel production is
probably going to benefit from future technological breakthrough.
For example, with new production technologies called "thin
layer" (because the semi-conductor is deposited as a very
thin layer on a plastic substract), the initial energy debt would
represent only 1 to 2 years of the future panel output, or even
less. This would remove a first significant obstacle to envision
a significant wpreading of this way to produce electricity. Let's
recall that the "initial energy debt" for liquid hydrocarbons
is around 20% : for every litre of oil that we consume (under
the form of diesel oil, petrol, etc) we have to burn teh equivalent
of 20 cl to extract, transport and refine it.

However, for the time being, it's
storing electricity that remains the weak point. We could of course
mass produce electricity with panels connected to the grid, but
if the name of the game is to get rid of fossil
fuels, or to cut rapidly the greenhouse
gases emissions, we'd better use hydroelectric or nuclear
electricity (see below). Furthermore, as solar production is intermittent,
it is not possible to conceive a system "100% photovoltaïc
solar" without any storage device or complementary production
means. And when the intermittence is very pronounced (which is
the case with photovoltaïc solar : all production during
daylight, nothing at night) the complementary production means
are gas or coal, which is not optimum to meet the two above mentionned
objectives !

If photovoltaïc solar systems
are considered mainly for self consumption, it is necessary to
store : with the available technologies (lead-acid batteries),
we should have a couple tonnes of batteries in every house (see
calculations on this page), what would
require considerable amounts of resources. Putting solar panels
everywhere today remains difficult to conceive without a significant
improvement of storage devices, not
to mention that the average power delivered by a set of batteries
is much better suited to domestic applications than to industrial
ones (in France the industry consumes 50% of the total, roughly).
And regarding storage, alas, breakthrough is less in sight than
for production.

***

Is
photovoltaïc solar a good idea to mitigate global warming
?

Well, it all depends on the way
electricity is produced in the country.

If photovoltaïc solar is
definitely much better than electricity produced with coal or
gas fired power plants, it remains a higher emitter per kWh than
the two real "CO2 free" modes which are hydroelectric
and nuclear (wind power is totally marginal
and bound to remain so for a while). In France, for example, replacing
nuclear power plants by photovoltaïc solar would not lead
to a savings on greenhouse
gases emissions, on the opposite.

But it is perfectly defendable
to use such a way to produce electricity to help "pulling
out from oil", or rather "pulling out of coal".

***

Is
it economical ?

The average price of the kWh (between
30 and 60 cents of Euros depending on the sources ; without taking
storage into account) is about 4 times higher than the price of
domestic electricity in France. But :

if electricity generated out of coal, gas or oil had to support
a carbon tax, the price
of the kWh (just after production) could jump to this much,

for the last 20 years, the price per kWh has decreased accordingly
with the installed power, and the technological breaktrough in
sight make legitimate to consider that this decrease will go
on for a while ; a division by 3 of the price per kWh around
2020 is considered likely by experts. Taking into account the
externalities of fossil
generated electricity, photovoltaïc solar could therefore
become competitive with "fossil electricity" around
2010 (but will not be compared to nuclear, which is another
good idea for the future).

This "quick calculation"
does not pretend to highlight an obvious solution to the double
threat posed by the greenhouse
effect and the fossil
fuel depletion. But it underlines that the magnitude of the
supply still allows to consider that "someday" (a day
not too remote, preferably, if we want to avoid big
trouble) we might get a significant energy resource for our
needs (that could be strongly diminished).

***

Is
it necessary to use photovoltaïc panels to generate electricity
out of sun rays ?

There are many other ways to generate
electricity out of sunrays :

it is possible to heat a fluid by concentrating the sinlight
on it with parabolic reflectors, the heated fluid then being
used to generate vapour that will in turn generate electricity
:

Basic principle
of concentration solar : a parabolic reflector (left white arrow)
concentrates sunrays on a tube carrying a fluid (right white
arrow). The fluid then heats a reservoir (now shown on the picture)
to several hundred °C ; this reservoir then serves as a heat
source to generate vapour then electricity.

The reservoir is
hot enough and insulated well enough so that the plant can continue
to generate electricity overnight.

The surface required to produce
a TWh is probably twice lower than for photovoltaïc solar.
Such plants are already economically competitve in highly insulated
zones (close to deserts for example). Instead of exporting oil,
some tropical countries could export electricity ! It is true
that it would not solve them problem of the energetic independance
of France, but we would partly solve that of climate
change, which is not without interest....

It is also possible to concentrate
the sunrays with a forest of plane reflectors, fixed on the ground,
that converge the light to a tower installed in the middle.

It is possible to built a high tower painted black, in which
the air in inspired at the bottom, heated in the black - hence
hot - tube of the tower, and rejected hot at the top. This generates
a permanent ascendant air flux in which windmills are placed
(Australians have built such a prototype).

***

Is it possible to do "something else"
than electricity with solar energy ?

There is another way to use solar
energy, which is technogically mature enough to be widely used
: it's to get heat out of the sunrays. In this case, instead of
putting on the roof a panel designed to produce electricity, the
idea is to put a panel that produces hot water that will be used
for sanitary uses, or for heating. The yield of such a device
is three times better than that of a phtovoltaïc panel (that
means that when producing hot water, a panel captures 30% to 40%
of the incident solar energy).In
favourable conditions, such a device allows to save about 50%
on the energy spending that corresponds to heating and sanitary
hot water.

As these two items (heating and
sanitary hot water) represent more than 30% of the total energy
consumption in France (including domestic and office use), a wide
use of thermal solar would allow to save 10 to 15% of our total
energy consumption (to give an element of comparison, in
1999 the Danes got 1,5% of their total energy consumption with
wind power). A "massive" call on thermal solar,
coupled with an important insulation of ancient houses and offices,
might lead to a rough 25% saving on the national total.

The conclusion is therefore that
thermal solar has a true potential to contribute to savings on
fossil fuel consumption without a significant loss on comfort.
Furthermore, heat storage has not been much investigated in our
modern societies (much more investigations have been made on electricity
storage), and if we could manufacture a well performing seasonal
storage device (for example with well insulated underground reservoirs
of salted water with stones), we could probably get much more
out of this source.

And a vast plan aiming at getting
all houses well insulated and as much heated with thermal solar
as possible would be a very good thing for employement : today
2 million people buy a new car each year, worth a little less
than 15.000 euros on average. If "pulling out of fossil fuels"
became the first priority of our economy, one can imagine that
half a million to one million households rather buy a complete
renovation of their lodging, with a thermal solar heating among
other things, so that all lodging is renovated in 40 years. If
the unitary cost of every operation is about 15.000 euros, it
would create an economic activity worth 30 billion euros per year
in France, benefiting for the most part to plumbers, bricklayers,
and small industrial companies.

***

And
even hydrogen....

With solar power plants (such
as concentration solar plants, see above), one can also consider
to produce hydrogen, through water thermolysis (it is then a totally
renewable resource) or the thermolysis of a mixture of water vapour
and methane (this way to produce hydrogen does not avoid greenhouse
gases emissions, but still much less than producing
hydrogen out of methane cracking). However, as the only places
where a large scale hydrogen production out of solar energy are
tropical deserts, we would then be faced to the problem of transportation
of hydrogen over long distances, which is not obvious to solve.

Indeed, the amount of energy that
must be spent to store and transport a gas is roughly the same
whatever the gas is. It means, in concrete terms, that it takes
the same amount of energy to transport and store 1 cubit metre
of gaz, whether it is methane or hydrogen, when the energy freed
by combustion is much higher, per volume unit, for methane than
for hydrogen (there is a 3 à 1 ratio: 10 kWh/m3 for methane,
3,3 kWh/m3 for hydrogen).

This means that the efficiency
of storage and long distance transportation of hydrogen is probably
bound to remain forever "not very good" for physical
reasons. Still, this way of producing hydrogen might be very interesting
for well insulated countries, that are quite numerous !

And biomass ?

At last, the sun enables to growth
of trees... that therefore constitute a stock of solar energy.Is it interesting to use it to produce
electricity ? The gross energy output per hectare is about 5 tonnes
oil equivalent in the bast cases (in european countries), that
is about 60 MWh (1 tonne oil equivalent = 42 billion joules =
11600 kWh). This amount corresponds to the annual growth of the
wood, of course, not to what would be obtained if all the wood
was logged and burnt (but then it is not renewabel at all !).
Knowing that the efficiency of a power plant is 40% to 50%, it
requires something like 500 km2 of forests to produce 1 TWh (that
is 1 billion kWh).

If the anual electricity production
in France (about 550 TWh) was totally made out of wood, we would
need for this purpose about....50% to 100% of the surface of the
country ! We find here figures that are as "irrealistic"
as those obtained for biofuels.

But the potential of wood for
direct heat production is much higher. As the enery spending for
heating is about 0,75 toe (1 tonne oil equivalent - or toe - is
worth 11,600 kWh) per French person, and as the efficiency of
a a good central heating system is 60%, heating the whole country
on wood would require 120.000 km2 of forest land (20% to 25% of
the country). We could do so, providing we accepted not to have
any more construction wood, or not more meat (each usage requires
20% of the surface of the country).